Non-aqueous secondary batteries, which are represented by lithium-ion secondary batteries, are widely used as main power sources of portable electronic devices such as a mobile phone and a notebook computer. Their application has been expanded to a main power source of an electric vehicle or a hybrid vehicle, a power storage system for night-time electric power, and the like. Along with the widespread use of non-aqueous secondary batteries, ensuring stable electrical characteristics and safety becomes an issue.
Separators have significant roles in ensuring the safety of non-aqueous secondary batteries. In particular, in terms of a shutdown function, a polyolefin microporous membrane which has a polyolefin as a main component is used in the present circumstances.
However, a separator composed of only a polyolefin microporous membrane has a possibility that the entire separator is melted (so-called melt-down) when the separator is exposed to temperatures exceeding a temperature at which the shutdown function is exhibited.
In addition, a polyolefin resin shows poor adhesiveness to other resins or other materials, and thus adhesiveness between a polyolefin microporous membrane and an electrode is insufficient, which may result in capacity loss or cycle characteristics deterioration.
A provision of a porous layer containing various resins and fillers on one or both sides of a polyolefin microporous membrane has been proposed in view of improving heat resistance of a separator or improving adhesiveness between an electrode and a separator (for example, see Patent Documents 1 to 7).
In particular, as a technique to improve adhesiveness between an electrode and a separator, a separator in which a porous layer which has a polyvinylidene-fluoride resin (hereinafter, referred to as “PVDF layer”) as a main component is formed on a polyolefin microporous membrane is known (for example, see Patent Documents 1 to 3).